Measurement of Young's modulus of nanocrystalline ferrites with spinel structures by atomic force acoustic microscopy
Introduction
Thanks to their particular physical properties, thin films of nanocrystalline ferrites with spinel or hexagonal structure could be used in magneto-optic and magnetic recording systems. They are also candidates for use in advanced applications, such as near-field magneto-optic recording systems. Nanocrystalline ferrite films exhibit high magneto-optic effects, a reduced optical absorption at wavelengths between 400 and 500 nm, and a high coercivity that allows small-sized magnetic bits in magnetic storage media. They are also chemically stable and have a high mechanical hardness. Their technological applications depend largely on the control of their synthesis and their nanostructure. Particularly, in the case of nanocrystalline ferrites with spinel structure, it has been shown that the coercivity Hc is not only correlated with the chemical composition, but also with the grain size [1] and the quenching temperature under oxygen pressure [2]. For a given grain size in the 40–80 nm range for which Hc is maximal, Hc depends on the cation distribution and on the point defect concentration in the two sublattices of the spinel structure. Furthermore, it has been demonstrated by theoretical simulation [3], XPS spectroscopy, and X-ray diffraction [4] that the coercivity is related to the chemical gradients induced by the oxidation process in the γ-phase which in turn produce mechanical stresses. Our aim is to show experimentally that the elastic behavior of the nanocrystalline ferrites is related to the occurrence of chemical gradients and stresses in the ferrite films as a function of the quenching temperature, which in turn influences their coercivity Hc[5].
In this paper the measurement of the Young's modulus of thin films of magnetite Fe3O4+δ and Cu–Co–Mn ferrite Cu0.7Co0.3Mn0.75Fe1.25O4+δ, where δ is the deviation from the stoichiometry, is reported as a function of the quenching temperature under ambient oxygen pressure. To measure Young's modulus, dynamic Atomic Force Microscopy at ultrasonic frequencies is employed. This technique is also called Atomic Force Acoustic Microscopy (AFAM). AFAM allows one to measure the contact stiffness of the specimen with a spatial resolution determined by the contact radius which is below 50 nm. From the contact stiffness the Young's modulus is calculated using an inversion algorithm.
Section snippets
Experimental apparatus
The experimental set-up to measure the contact stiffness is shown in Fig. 1. The basic idea is to excite the cantilever of an atomic force microscope into flexural vibrations when its tip is in contact with a sample (Fig. 1). A more detailed description of AFAM can be found elsewhere [6]. The frequency of the eigen-modes of the cantilever depends, amongst other parameters, on the stiffness of the tip-sample contact and on the contact radius, which in turn both are a function of Young's modulus
Evaluation of data
The characteristic equation allowing the evaluation of the stiffness of the sample from the measured contact resonance frequencies is discussed in detail in [6], [16], [17]. The important points are summarized in what follows. When the cantilever is in contact with the sample surface, vertical and lateral forces act on the sensor tip. Both forces are nonlinear as a function of distance between the sensor tip and the sample surface. For sufficiently small vibration amplitudes of the cantilever
Materials
The ferrite thin films were directly deposited onto cooled glass substrates by RF sputtering of a spinel oxide target. The samples were treated at 450°C in a mixture of nitrogen, hydrogen, and water in order to obtain the stoichiometric phases per mole of ferrite, i.e. δ=0, and to obtain films with a thickness of 200 nm [20]. The thin films of magnetite and Cu–Co–Mn ferrite were oxidized between 50 and 550°C for 30 min and then quenched in ambient air. The average grain size for Fe3O4 and for γ-Fe
Results
In order to obtain an overview on the nanocrystalline structure of the materials we examined, we made standard AFM topography images, one of which is shown in Fig. 5. It depicts a topographic image of γ-Fe2O3. These measurements follow work on other nanocrystalline materials published earlier [23], [24]. As can be seen, the nanocrystalline structure of the film becomes visible with an average grain size of 40 nm. The image comprises a surface corrugation not exceeding 20 nm in the peak-to-peak
Young's modulus of the stoichiometric thin ferrite films
To our knowledge, no measurements of the elastic properties of nanocrystalline thin films of ferrites in stoichiometric equilibrium have been reported. However, the elastic constants have been measured for various single crystals of ferrites [26], [27], and for ferrite materials prepared by sintering at high temperature with a grain size in the micrometer range [28], [29], [30], [31]. The values obtained for the Young's modulus E show that the differences originate mainly from the chemical
Conclusion
In this contribution, Young's moduli of nanocrystalline thin films of magnetite Fe3O4 and of Cu–Co–Mn ferrite Cu0.7Co0.3Mn0.75Fe1.25O4+δ have been measured as a function of the oxidation temperature using an Atomic Force Acoustic Microscope. From the shift of the resonance frequencies of the cantilever whose tip is in contact with the sample relative to the resonances of the free cantilever, the Young's modulus is determined locally with a lateral resolution of approximately 10 nm. An
Acknowledgements
One of us (E.K.) thanks the European Union for financial support within HCM/TMR program, project “Microacoustics and Atomic Force Microscopy”. He thanks also the Graduate College “High-Performance Materials” at the University of Saarbrücken for support following the HCM project. The major part of the research reported here has been supported by the Volkswagen Foundation. We also acknowledge a generous donation by A. Brück, Brück GmbH, Saarbrücken-Ensheim, which was used for instrumentation
References (41)
- et al.
Mater. Chem. Physics
(1997) - et al.
J. Phys. Chem. Solids
(1996) - et al.
J. Phys. Chem. Solids
(1998) J. Cryst. Growth
(1996)- et al.
J. Magn. Magn. Mater.
(1996) - et al.
Mater. Lett.
(1992) - et al.
J. Magn. Mater.
(1997) - et al.
Mater. Lett.
(1999) - et al.
J. Nucl. Mater.
(1995) - et al.
React. Sol.
(1986)